Gas turbines often include a compressor, a number of combustors, and a turbine. Typically, the compressor and the turbine are aligned along a common axis, and the combustors are positioned between the compressor and the turbine in a circular array about the common axis. In operation, the compressor creates compressed air, which is supplied to the combustors. The combustors combust the compressed air with fuel to generate hot gases of combustion, which are then supplied to the turbine. The turbine extracts energy from the hot gases to drive a load, such as a generator.
To increase efficiency, modern combustors are operated at temperatures that are high enough to impair the combustor structure and to generate pollutants such as nitrous oxides (NOx). These risks are mitigated by directing pressurized air supplied from the compressor over the combustor exterior, which cools the combustor, before premixing the air with fuel to form an air-fuel mixture, so as to generate lower levels of NOx during combustion.
For these reasons, the combustor typically includes a flow sleeve that defines an annular passageway configured to receive the pressurized air discharged from the compressor. Specifically, the air impinges against the transition duct and combustion liner for cooling purposes. The air then travels in a reverse direction through the annular passageway toward the combustor cap assembly, which houses at least a portion of the fuel nozzles. Often, a portion of this air may be diverted from the annular passageway and into the cap assembly to provided cooling to such assembly. For example, a downstream plate of the cap assembly may be exposed to the high temperatures of the combustion chamber. Thus, the downstream plate is normally cooled with air diverted from the annular passageway through openings in an outer wall of the cap assembly. The diverted air impinges against and passes through the downstream plate into the combustion chamber. Thus, the diverted air is not pre-mixed with fuel, which exacerbates NOx generation.
Typically, the air traveling through the annular passageway experiences pressure loses. Due to these pressure losses, an increased amount of air is needed to cool the cap assembly, resulting in a lower percentage of premixed air in the combustor. Also, the air pressure through the downstream wall may not be sufficient to overcome a dynamic pressure wave that is present in the combustion chamber due to flame instability and/or other combustion dynamics. Specifically, this dynamic pressure wave may exert a pressure on the downstream wall that impedes or stops the cooling flow, causing the downstream wall to overheat and potentially fail.
Accordingly, a system for supplying pressurized air to the cap assembly that allows the pressure within the cap assembly to be increased would be welcomed in the technology.